At 01:21 PM 10/20/2006, Robert G. Brown wrote:
>On Fri, 20 Oct 2006, Jim Lux wrote:
>>>> What in the world would it do that
>>>couldn't be done far cheaper back on earth, or in orbit?
>>>>Anything that requires realtime control with a time scale smaller
>>than the propagation delay of the comm path.
>>Yeah, but as I was saying that would be ``nothing'' with comm latencies
>of 0.01 seconds or better from geosync Mars orbit. I can't do payload
>cost in my head for a Mars run, but I'm going to guess that whatever it
>is, it will cost several orders of magnitude more per kilogram to
>deliver anything at all gently to the surface of Mars than it would to
>deliver it to a geosync Mars orbit.
Oh.. aerostationary orbits... Then, if you want to cover the entire
globe, you need 5 satellites (if they're in circular orbits). And,
the delay will be longer than 10 milliseconds, depending on the
orbital height. I forget the numbers for Mars, but for Earth, Clarke
orbit at 36,000 km, the delay (round trip) is at least 240 ms.
And of course, you'll need to launch 5 widgets.
To a first order a kg on Mars costs about 10 times as much as a kg in orbit.
>So any SANE mission to Mars, rather than a silly one, would be a two
>step process -- build and deliver an orbital platform and maybe two more
>small relay satellites (the latter don't need to be there for the first
>ground mission). On the orbital platform put your cluster (if one is
>needed) and high powered earthlink antennae and radio booster. Solar
>power abundant and not attenuated by Mars' atmosphere or inconvenient
>things like "nighttime", plenty of 3 degree Kelvin sky for blackbody
>cooling if you shield the cooling coils with power collection panels on
>the other side and/or reflective foil. No attenuation of the Earthlink
>signal. Point a nice reflection dish antenna down to the ground, match
>it with a nice uplink antenna on the ground. No need to worry about FCC
>regs, use the entire bloody spectrum if you like, maybe use a visible
>laser or a maser to encode the channel, should be able to get GHz per
>channel on hundreds of channels at a guess or at LEAST GHz+ of net
>bandwidth on the uplink.
And this is exactly what Chad Edwards (and others at JPL) have
proposed for the Mars network. We even had a proposed mission, Mars
Telecommunictions Orbiter (MTO), to do just these things (well.. no
cluster), but it has been postponed.
>Personally, I'd have put up a minimum of three orbital platforms up
>ANYWAY before even attempting what they've already attempted in terms of
>landing mars rovers. Geosync on Mars is a lot closer (in km) than
>geosync on Earth, and if the military and weather programs have given us
>nothing else, they've given us simply gangbusters orbital cameras.
Doesn't even really have to be Aerosynchronous or
Aerostationary. The real criterion is that you are far enough away
that you get decent coverage without needing to have the landed asset
antenna pointing too close to the horizon.
>So long before the next Mars Rover type mission, they need to give us
>"Google Mars",
They have Google Mars..
http://www.google.com/mars
most of the imagery that's detailed is from Malin Space Systems, and
is meter scale resolution (enough to see your house, if it were on Mars)
>but from much closer in and with the higher resolution
>better optics and less atmosphere can yield. I would make the whole
>damn planet available online -- I'm not kidding about Google Mars -- and
>update the images and views available agressively (adding time as a
>dimension as well as aspect/angle of view where possible).
MRO's HiRise camera has sub-meter resolution images. You can
actually see the shadow of Opportunity's pan cam
mast. http://mars.jpl.nasa.gov/ Take a look at the press release
for 6 October... you can see the rover sitting next to Victoria
crater (and wheel tracks)
I don't know if Google Mars is ingesting these images yet
And, they DO use overhead imagery and stereoscopy to generate terrain
models. Not to mention that the rover does it itself using the nav cams.
>>Signal processing for seismic or subsurface radar is another possibility.
>>>>>> In fact, it is "free" -- available at opportunity
>>>cost on a communication channel that has to exist no matter what, and
>>>bandwidth in that channel is similarly cheap, all the way to Earth.
>>>>That channel is not cheap, nor is it available 24/7. For instance,
>>you might be on the far side of Mars, with no direct path to
>>Earth. There are a few relay satellites available (most with very
>>skinny pipes) and for the current ones at least, the comm pass
>>lasts <20minutes. Putting a 24/7 communications infrastructure in
>>place around the moon or Mars is a pretty expensive proposition
>>(many billions of dollars). It might well be that spending a
>>As I said, that channel has to exist no matter what, so its cost delta
>is irrelevant.
But the channel bandwidth does cost something, and incremental
bandwidth is fairly expensive (translates directly into watts and
mass, fairly linearly) Doubling the bandwidth takes twice the power
and mass. For most missions, there isn't any plan for excess
capacity (there always IS, because things usually work out better
than expected.. we in the space business tend to be pessimistic about
performance predictions). You would be directly trading
mass/power/volume to do processing insitu (either ground or in orbit)
against mass/power/volume to just send the unprocessed data.
I haven't run the numbers (perhaps I will, over the weekend) but
it's conceivable that insitu processing might be more cost
effective. Especially so since Moore's law helps the processing
burden, but does nothing for bandwidth (we're already coded and
processed for comm to within hundredths of a dB of the Shannon
limit). As computers get faster and lower powered, the trade
definitely shifts in the direction of local, not remote processing.
> Also, the advantages of putting as serious network of
>satellites around Mars are far greater than just establishing a high bw
>channel from surface to orbit and orbit to Earth, although those are
>definitely advantages -- as noted before with Google Mars.
But that's the "funding for infrastructure" vs "funding for direct
science" challenge we always face. Much easier to get the latter,
especially since the "quanta" is smaller.
> Finally,
>you've got the arithmetic exactly backwards -- it almost certainly costs
>ten times more to lower mass gently to the ground on Mars than it does
>to put it into Mars orbit.
I agree.. I may have mistyped...
>So no, I don't think that there is
>any credible way that it isn't cheaper to build orbital infrastructure
>first and THEN go down than to try to just go down. The latter may sell
>to the public better, of course, but hey...
>>>run in the $100M to $750M range, and tend to stand alone, although
>>part of an overall program)
>>I believe that this is a big part of the problem. Putting a bunch of
>really well equipped satellites in orbit around Mars isn't nearly as
>sexy as putting down a single rover, even if it actually costs less and
>teaches us more and gives us the eventual chance of putting down a rover
>that can leave most of its "brains" up in the sky, leaving more room for
>interesting payload.
Precisely
>>You'd be surprised... for batch processing type applications, where
>>it's ok to wait til the next satellite pass for the next data
>>bolus, the trade is easy.
>>What next satellite pass? Equatorial geosync, please, right over or in
>easy line of sight with the rover site. Mars' atmosphere is so thin
>that cutting through it on a tangent is still easier than cutting
>straight out of Earth's, so this really should work all the way to the
>poles. Three such satellites really ought to span the globe as far as
>communications is concerned, although of course more would be better. A
>handful of satellites in polar orbits for doing mapping wouldn't hurt as
>well.
But you're back to the needing 5 satellites (and not all of them need
computational power) and that's a hard sell.
>Really, one could send up just ONE big payload to Mars orbit and ship
>out to different end-stage orbits along the way (where it is cheaper) --
>this isn't about sending five or ten flights, just one big flight with
>five or ten payloads.
This has been proposed.
>>SO far, no patron has stepped up with $200M to send something to Mars, yet.
>>Well there are those hardy optimists that have dropped $200M trying to
>build a commercially viable low earth orbit transportation company. The
So far, none have been hardy optimists for Mars. Probably that
commercially viable thing.
>In the research, communications, weather, observation, and military
>domain low earth orbit is a very useful place to be. As far as ANYTHING
>ELSE -- e.g. manufacturing something of actual value, I have yet to see
>credible evidence that orbit is good for much. The moon ditto, unless
>it proves to have some raw materials in abundance that are scarce on the
>Earth. Mars triple-ditto. And I say this with great regret, as a
>reader of nearly every decent SF novel ever written on the subject, many
>of them many times over...;-)
>>That doesn't mean that I think we should abandon these projects -- au
>contraire! It does mean that I think that there needs to be a long term
>plan in place, that it needs to be recognized at the outset that the
>investment required is for pure science with no expected payoff (however
>often pure science has created UNexpected payoffs beyond our wildest
>dreams), and that in the long run the projects DO need to break even or
>win a bit in return on investment compared to, say, ending world hunger
>or eliminating war or achieving universal literacy.
You are, of course, preaching to the choir here...
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875